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[Reply #15 on:]

Well measure all the voltages and supply current both before and after activating the chip-enable line.

Your PCB layout seems poor, you have long thin traces where there needs to be short wide traces (to both capacitors). 
Short = a couple of mm if possible.  There's no point using low ESR and low ESL capacitors and then putting them at
the end of 10mm of thin PCB trace, that defeats the object.   A 2-layer board is going to be a lot easier to get working
as the ground return path can be directly underneath, keeping stray inductance right down.

[Reply #16 on:]

Why not try to use an NCP3063 instead? It would be easier. (it's a jelly bean part, very simple, easy to get going)
I just whipped a quick 3.3V to 15V converter using parts in my junk bin... on a breadboard.
3.3V to 5V should be even easier, just change values of 2 resistors.

If your design/wiring/components are right, it should work even though it's built on a breadboard.

When you put it on a proper PCB and layout is optimal, you'll see increases in efficiency. Power Out / Power In
But it should still work even when built on a breadboard.

[Reply #17 on:]

Regarding the layout: I have been going at this from the wrong way. I mentioned earlier I couldn't use the layout in the datasheet, it appears I just didn't understand the simplicity of it: I only need to make a small modification for the different N-channel mosfet to make it meet my demands, it fits most all your recommendations, apart from the fact that it is single sided. But since they designed it, I suppose it will work. I'll post the design as soon as I am done battling the Eagle.

NCP3063... why on earth did I not discover this chip while searching for a good dc booster ? I searched Farnell for quite some time and apparently did not find it, though they DO have it. [time passes while browing farnell and reading data sheets...] It is because the output voltage is rated as 40 Volts, while I was searching for 5 Volts output. With external components you seem to be able to regulate voltage. I do not understand enough of dc boosters to have seen this one coming. Is this normal for booster chips ? Because then I will have to review the entire bunch of them.

What I need for my project is a booster that delivers 5.0 Volts from 3 AA batteries with peak rating for current of max 1 Amps. I prefer it to be (1) easy to build, (2) as cheap as possible, (3) with a preference for DIP parts. The NCP3063 seems to fit this nicely. Is the NCP3063 my best bet then ? If so I will read into the tytpical application circuit, check my supplies and order what is needed, so I can whip one up.

On another note: I already have all the components for the NCP, so I will try to build one, even if the NCP3063 will be the one I would like to use in larger quantities for projects, just for the sake of building it (what else would I do with these components).

[Reply #18 on:]

DC/DC converters are not magic boxes. They're also not 100% efficient. On a good day, expect 80+%, and in some configuration 50% or so.

5V at 1Amp from a AA batteries may be expecting too much. The converter can only produce output power from some percentage of it's input power source. In your case (3) AA batteries (which is how many mAmps?). Don't forget that.

The NCP is very forgiving. Even if your short the output, output voltage clamps down to 1.25Volts or so. Just reset the power (unplug/switch on/off the input voltage source) and your DC/DC converter is back up and running again.

It is because the output voltage is rated as 40 Volts, while I was searching for 5 Volts output.

No, 40V is the max. input voltage for the NCP chip. From there, you can adjust it down or boost it up, or boost/invert it.

[Reply #19 on:]

To maybe quantify and clarify what vasquo said at the beginning.  Lets say you want 5V/1A output and your cells are at 4V.  That means that your cells will have to supply 5/4 = 1.25A and that would be at 100% efficiency.  Well that isn't happening, so at say 80% your cells would have to supply 1.25/.80 = 1.56A to get 1A output.  That's going to be tough on battery life so you might consider more cells and a buck converter.

[Reply #20 on:]

The cells I use are 1.3 - 1.5 Volts, 2700 mAh

I understand the power conversion explanation. I might have to be a bit more clear about what I try to accomplish:

I'm building a laser tagger, which is a form of optical tag with infrared beams. Normal usage is 0.05 Amps for the tagger. Switching on the backlight of the lcd will draw 0.30 Amps, but this will NOT be done continuously. The tagger has a couple of leds which light up if you've been tagged, together they will draw somewhere between 0.10 - 0.20 Amps. So 1 Amps is not what I need most of the time.
The reason I would like it to be able to peak to 1 Amps is because I would like to pulse my IR led for extra range. Typical on-times for the led are ~1 ms, about ~20 pulses. For all the other purposes, most of the time I will draw 0.05 Amps.

At the moment I'm using 6 AA cells with 7805 converter, which amounts in a dear loss of power. It has been suggested to use an LDO to conserve energy, I will apply that for all 6 cell units.

However, 3 cells is cheaper, takes up less space and brings down the weight of the tagger. So that's why I'm looking into that.

So, I understand now the NCP3063 can be regulated to yield 5 Volts, it's a robust chip (that comes in handy, because students will do the building of circuits), so if anyone has any other contenders to suggest I would like to hear, otherwise I will order it and some complementary components to make it work.

Forgive my ignorance, but I gathered from what I read that a Buck converter is like a step up converter, but it steps down. So the NCP can be used to build a boost converter, buck converter and voltage inverter, am I right ? In the case of a buck converter, what would be the advantage of a buck versus say, a LDO or 7805 ?

I appreciate all your input, typing out my thoughts and reading your responses helps with the understanding.

[Reply #21 on:]

Any LDO is a voltage dependent resistor where the resistive element is the pass transistor. The voltage differential like with any resistor is dependent on the current through it and the power dissipated in the regulator is that difference in voltage times the current through the regulator. With high I/O differentials this power can  exceed the load power quickly requiring a larger heatsink than the load device. Switchers take just enough current to charge the inductor and filter to the required voltage it is in how quickly the energy can be transferred that the power is saved. This is an overly simplified description of the switcher, but I think conceptually accurate.

Doc

[Reply #22 on:]

So the NCP can be used to build a boost converter, buck converter and voltage inverter, am I right ? In the case of a buck converter, what would be the advantage of a buck versus say, a LDO or 7805 ?

Correct. It can be used in those 3 configurations. The only caveat is in the Inverting configuration, efficiency is the lowest.

Buck vs LDO vs. 7805

The 7805 needs at least 1.25V differential between Vout and Vin. Any excess voltage is converted to heat.
The LDO solves this problem by offering a lower drop out voltage.
The Buck doesn't "burn" off excess voltage, but only switches on/off the duty cycle to charge the inductor, and the inductor supplies power to your load (via capacitors).

The disadvantage of the DC/DC buck is the output is not a smooth DC waveform (unlike the 7805 and LDO). Instead, you have a sawtooth type ripple. This can be reduced by choosing the appropriate L and C values (but there's always tradeoffs when you go bigger or smaller). This ripple can be reduced by hanging an extra LC filter at the outputs. But in your particular application, this ripple voltage may not be a problem and your project should still run just fine. 

For the NCP, try to switch it at it's highest freq 150Khz so you can use the smallest possible size inductor if weight/size is an issue for you. I've built DC/DC using the NCP on PC boards measuring 1"x1" (and this is using a PDIP chip, not the SOIC version). The PDIP chip has a higher temperature capability, plus it's easier to glue a heatsink on it's top if needed.

[Reply #23 on:]

Allright, this goes two ways.

Included from the NCP datasheet the original design of the evaluation board:





Now I don't have the exact same components, so this is my take on it:



I couldn't squeeze the small line underneath my diode, so it goes underneath my coil. Note: on the image my coil is large and circular, in reality it fits on the pads, so ignore the big circle, it does not create space conflicts. As far as I can tell the board is single sided, is that safe to assume? I have no idea what the 12 little white circles in a grid are on the original design.

So far my SMD design. If all is well I can have it etched tomorrow.

I have been looking into the NCP3063 as well and found a very useful document from OnSemi:

http://www.onsemi.com/pub/Collateral/NCP3063%20DWS.XLS

The excel sheet helps to select components. I entered my desired values and got some results:



Now this raises a few questions:
* a 10 uH coil will be good as well, since they say "minimum" ?
* do I need special (ESR / tantalum / ?) capacitors, or are any regular capacitors sufficient for this task ?
* D(Vf) is the diode forward voltage as far as I can tell, but they recommend 1N5819, which i now see is available in 0.6 Volts Vf. However, should I stray from this value, should I go higher of lower?

Cheers,

Jack

[Reply #24 on:]

Now this raises a few questions:
* a 10 uH coil will be good as well, since they say "minimum" ?

Here's the thing with the "minimum" disclaimer. While your circuit may work and give you the output voltage you're expecting, it may not hold that output voltage for higher loads.... i.e. at higher loads, you may start to see your voltage come down sooner.  Play with the values and test.

But, if you select a too big an inductor, for a given load resistance (current draw), your DC/DC converter may not startup.

* do I need special (ESR / tantalum / ?) capacitors, or are any regular capacitors sufficient for this task ?

You can use aluminum electrolytic, just make sure it's low ESR.  If you're breadboarding this to play around and experiment, you can use ordinary elect caps in your parts bin but when you buy your final parts, use a low ESR model.

A big Cout means lower ripple voltage. But Inrush current may be too high. Measure it during startup and see if you're comfortable with the values. i.e. at normal loads, your converter may be drawing 200mA, but at startup, it could shoot up to 800+mA for an instant. 

For some configuration, (like Inverting) using a smaller Cout actually was actually more beneficial (better performance, higher load capability) than a big Cout. But then ripple will be high.... what to do?

You can hang another LC filter at the output of your converter, and you can reduce ripple (and clean the output too very nicely) by another 50%!



* D(Vf) is the diode forward voltage as far as I can tell, but they recommend 1N5819, which i now see is available in 0.6 Volts Vf. However, should I stray from this value, should I go higher of lower?

What's important here is to use a schottky diode. You can't use an ordinary diode... it'll get hot. The 5819 will work or any schottkey diode (say you used another SMD part).


Other than the schottkey diode, you can use whatever parts you may have in your parts bin. Efficiency may not be as high, or ripple may be larger depending on the kind of parts you put together.  You can breadboard this and see the effect of your chosen part (use a scope, voltmeter, ammeter and a load).

Just remember, this is a game of tradeoffs when it comes to component selection. Depending on your chosen components, your peak efficiency may be at lighter loads, or heavier loads, you'll either get a lower ripple or higher ripple, lower inrush current, higher inrush current, failure to startup beyond a given load, or reliable startup everytime at your maximum load, size, and cost.  Test test test for your specific load and application.

[Reply #25 on:]

All right ! We have success !

We etched the board and got quite a good result:



Populating it resulted in this figure:



At first I didn't work, we seemed to have broken the coil, we replaced it with a new one and... 5.05 Volts ! Hooah !

So I attached a scope and did some efficiency tests.



This shows the image of the riple generated by the booster. The upward spike is generally about 0.3 Volts, the downward spike about 0.2 Volts. The booster has been tested without another RC circuit for filtering out this ripple. Results look good. Efficiency test results:



So indeed if V_in is rising a better efficiency is achieved. I then connected a laser tagger to the dc booster, it even ran @ 1.5 Volts. So today a great success, this looks really good, thank you all so far for your contributions, it really helped out.

Might be nice to mention all the etching and soldering has been done by 16 / 17 year old students, so they did a nice job. Needless to say they are quite happy as well this is working.

I will try to build a DIP version as well with the NCP3063, and design a smaller version of this board, should be able to make it a bit more compact.

One question rises: the soldering has been done with a normal soldering iron with small point, but the results look a bit messy, as you can see in the picture. Should we be able to achieve better results with a soldering iron ? Practice makes the master ? Any tips on how to achieve this ? Or should I try to to this with (a) hot air nozzle (b) reflow skillet (c) reflow oven. Mind you: we haven't got (a), (b) or (c), but we can try and find / build any. We do have the soldering iron, so that's what we used.

Cheers !

Jack

[Reply #26 on:]

but the results look a bit messy, as you can see in the picture.

If you have this board professionally made (visit oshpark.com), it will come with soldermasks so only the pads/lands will have the exposed copper, keeping all lead confined into those areas only. -- making for a clean appearance.

Great you got your converter working!